What doesn't work
I have just done a full, clean install of from the Big Sur Release Candidate (build 20B28) on to an external Fusion Drive I put together from a SSD drive and an external HDD.
So far everything has gone well except for ...
Installing the full suite of software for my Canon TS5060 MFPrinter/Scanner.
The full installer program has not been updated for Big Sur.
Currently I have to download the Printer Driver, the Scanner Driver, the Scanner Utility and the Quick Menu as separate items and install all separately.